A phenol resin as a thermosetting resin is widely used mainly as a binder for binding materials that become a substrate of a molded product to each other. Having excellent mechanical characteristics, electrical characteristics, and adhesiveness, the phenol resin is used in various fields.
The phenol resin is used for impregnation. Examples of uses in impregnation include a wet friction material, a prepreg, a laminate, a C-C composite, Fiber Reinforced Plastic (FRP), a coated abrasive, and the like. For use in impregnation, a resol type liquid phenol resin is generally used. Further improvement is increasingly required for the characteristics of the phenol resin for use in impregnation, and particularly, for the purpose of improving toughness, the improvement of the flexibility of the phenol resin has been required increasingly. However, though having excellent mechanical characteristics, a hardened material of the general phenol resin has a property of being hard and brittle, so the resin is not necessarily excellent in flexibility.
Therefore, as a method of solving the above problem, an attempt at improving flexibility by using drying oil or the like as a modifier in a reaction for synthesizing the phenol resin has been examined (for example, see PTL 1).
However, such a modified phenol resin shows a marked decrease in strength after heat history, and has a problem of a short cycle life.